Radio receiver noise reducing circuit



1952 c. WASMANSDORFF RADIO RECEIVER NOISE REDUCING CIRCUIT Filed July 23, 1947 INVENTOR CARLTON WASMANSDORFF BY Mm, IUO' kut-41 8,5011? ATTORNEYS Patented Feb. 19, 1952 UNITED. STATES immo-imcmv'nit NOISEREDUCING' cinema- Carlton wasmansdorm Bentemncaim Applicationifll'y 23, 1947, sens-1 N 1 This inventionrelates to electrical circuits-and devices for reducing noise appearing atthe out= put-of 'aradioreceiver; a carrier frequency amplifier, or other amplifier which-is followed by: a

demodulator, the output beingderivedfromthe demodulator.

More particularly: the invention combines the reductionof noise by balancing itout inthe signali channePwitli'the-further reduction-oi the remainingnoise by limiting the--magnitude of peaks which are passed-to-the' in-put of the audiocfree quency amplification apparatus or other. device responsive to signal frequencies.

An objectof the inventionis to provide comprehensive noise reduction by first balancing iout noise in the signal channel by' opposing it with corresponding noise appearing in one or more noise chainnels, the balancing being effectedxcby combining the output of a signal detector with the-outputs of' one or more'noise detectors andat; the same timederiving a unidirectionallcontrol:

voltage whose magnitude varies inaccordance with the average amplitude of the signal :itobe' demodulated and utilizing this controlirvoltageito establish a varying limiting "thresholdin .anram' plitude limiter which in turn z-removes :undesired noise peak-s appearingin thesi'gnal afteridetection and noise balancing:

The no-isebalancing facilities may :be lofzf the type illustrated in my copending application; Serial No. 531,047, filed April 14, 1944, now aban doned, which utilizes-a twochanneli=arrangement and which may be characterizedas a frequency.-

modulation discriminator operatingas; anizzamr-y plitude-modulation detector, or it may tal'xerthe form of the multichannelarrangement illustrated peak limiting device a single actingitype as shown in my copending application Serial No. 466,077,

filed September 29, 1942, now'Pa-tent No. 2346565,;

granted August 10, .1948, may be sntiliiied, orrztiie limiter may be of thefull-wavetype .as exemplified in my copendingv application, Serial No. 738,999,- filed April 2, 19457,, now abandoned.

other convenient form .of noise balancing ,arrangement may be used which will yieldua unidirectionalcontrol voltage whose magnitude varies in accordance with the averagesignalintensity and any convenient form of limiters maybe utfl ized .in which this unidirectional control voltage establishes a thresho'ldbelow which no limiting action occurs in such manner that "for "weak" signals or low carrier level at the detector --iii put the maximum voltage passed by the limiterwill in mycopending application Serial No.1677.,45;5;; filed June 18, 1946, now abandoned; :inwhich two separate noise channels are utilized; For the 2 be greatlyireduced and: for: stronger zsigna-lsethe maximum: voltage twill 2bemorrespondinglyy:in-

creased; :=.suhstantial1y in-accordance with then averages-signalrstrengthzz :The viin ventiomalsosproviides. facilities; iorisder rivingeaneadditionaliicontrol voltagev which; alwaysarsnbstantiallyi proportional to: thee signal strengthrfcr ruse sin: circuits; external 1130:: those; of, the zinvention such as automatic n-vnlume acontrol circuitsiused toiicnisitralithe;iaicnnlificatien:of; fame plifymgecircniilfi v 1:2receding-i the 'muits. "of the invention. t-It'iis-iaifeature-crane mention that thisicontrclw volta esisnelatively: ineetfr mt he effefitsiflfi noise-sand itliatwtbis volta e s, haree fore:lusable-inicircuits where noi e effects would be. objectionable:

A1 further objectloffitheinvention is. temr ide. in: combination with; the facilities descr bed the .iprecedi-ngmarasraphs,= a uelch neutra s edit-actuated hma men ion at. the u idir ctional ccntrol'voltage; nAsini-ithe-icasevof the external.

control-voltagel-gthetsquelchicontml ,circuit ..is likewise relativelvatnee a-vfrom' thei-effectsaoi nois and:

intenmediate frequencytinputato (the invention appliedratnterminalli, thesother.inputiterminai being groundt lnwonder to pizovide correctbiasing potentiaisttor the eontrolsridsof 'nentodes 3 A- andfiiltheaoutnuticircuit oi. .thecarrier frequency or. intermediate frequency amplifier from which.

the inputto "terminal lglistderived should. include directscurrentcontinuity. I Obviously. -conven tionalv and. well; konwn. alternative I methods .pf/

providing grid biasmaybe utilizedsitdesired, ,blut

foriconveniencecthe. arnen ementssillustrated are consideredito be preferable;

C nductor. 6 connects the control. rids of v th tl'ireeapliljl'tofdesi, Wand Still, multiple to terminal I sothagt? an threer'eceive. the same input potentlal. 'A fixed gridbias for pentodefi is-derived fromic'athode resistor! audits-associated by"- pa'ss -capacitor--8Z 'Tli cathcdes e13:

neetea i-n muitipieanu a comm g-gr a bias isuc anode potential for pentode is derived from terminal 22 through the primary winding of transformer 23 and conductor 24.- Pentodes 3, 4 and 5 are arranged to act as amplifiers so that the common input applied in multiple to the grids of the pentodes will appear separately'at their respective outputs.

Transformer I2 is tuned by capacitors 25 and 26 together with damping resistor 21 to have a band pass characteristic which approximates that of the amplifier from which the common input to terminal I is derived. The pass-band of transformer l2 and its associated tuning capacitors is such that its central frequency is substantially identical with that of the amplifier with which the invention is used and the sharpness of tuning is determined by the efficiency of the associated components and the value of damping resistor 21. Similarly, transformer 29 is tuned by the capacitors 28 and 29 to have a pass band in which the central frequency is higher than the central frequency of tuned transformer I2. Likewise, transformer 23 is tuned by capacitors 39 and 3| to have a central frequency lower than the central frequency of tuned transformer I2. Diode 32 is connected to deliver a negative potential obtained by rectifying the output of transformer I2 and a portion of the current resulting from this potential flows along conductor 33 through the secondary of tuned transformer l2 and resistors 31, 38 and 39 to ground.

A capacitor 36 by-passes the carrier frequency components to ground. The negative direct current potential of conductor 33 is also applied through resistor 34 to the control grid of triode 35, and the efiect of this potential will be described later.

The two diodes 49 and 4| are both connected to deliver a positive potential with respect to ground instead of a negative potential as in the case of diode 32. In the case of diode 49, the positive potential passes along conductor 42 through the secondary of tuned transformer 29 and resistor 43 to conductor 44. A similar positive potential from diode 4| passes along conductor 45, through the secondary winding of tuned transformer 23 and resistor 46 to conductor 44, where-it joins the output of diode 49. A portion of the current resulting from this potential flows I to "ground through resistor 39; Components of carrier frequencies are by -passed to ground through capacitor 4! in the case of diode 49 and capacitor 48 in the case of diode 4|.

It is to'be noted that the current flowing in resistor 39 is the difference between the current produced by'the negative potential of diode 32 and the total current produced bythe two positive potentials produced by the diodes 49 and 4|. This bucking action causes the combined demodulated outputs derived from the high channel and the low channel to oppose thedemodulated output of the signal channel. Because noise components predominate to a greater extent in the high and low channels of tuned transformers 29 and 23 respectively, these noise components tend to balance out corresponding noise components appearing in the signal channel. By adjusting the value of resistor I9, the grid bias of pentodes 4 and 5 may be correspondingly adjusted and an amount of amplification obtained which yields optimum results from a noise balancing standpoint. The effectiveness of balancing is also determined by the tuning and pass-band characteristics of the three tuned transformers I2, 29 and 23. Preferably, the tuning of transformers 29 and 23 is such that the center frequency of tuned transformer 29 is somewhat higher than that of tuned transformer I2, and the tuning of transformer 23 is somewhat lower by substantially an equal amount, the pass-band characteristics being approximately the same for the three transformers.

Other tuning arrangements may also give satisfactory results, and in some instances it may be desirable to omit one of the noise channels entirely. For instance, pentode 5, and tuned transformer 23, and diode 4| might be omitted together with the other associated circuit elements such as tuning capacitors 39 and 3|, damping resistor 49, by-pass capacitor 48 and resistor 46, thus leaving a single noise channel to oppose the noise components in the signal channel. This, of course, may require readjustment of the values of other circuit elements so that the output of the single remaining diode will be sufficient to produce the desired noise balancing action.

At conductor 59, a potential appears which is produced predominantly by diode 32 and which is therefore negative with respect to ground. A current caused by this potential flows through resistors 5| and 52 to point B, and in the case of Fig. 2, through diode MI in its conducting direction and thence through resistor 292 to ground. In the case of Fig. 3, current from point B divides into two branches, one branch flowing through diode 39I and adjustable resistor 393 to ground and the other branch flowing through diode 392 and adjustable resistor 394 to ground.

The circuit constants are so proportioned that the voltage to ground appearing at point D is negative and has superimposed thereon the demodulated signals. The time constant of resistor 5| and capacitor 53 is such that the resultant filtering action effectively removes the signal components and is also such as to prevent response to short bursts of noise. This filtered current is delivered through resistor 52 to point B and thence either to Fig. 2 or to Fig. 3. The current thus filtered constitutes a unidirectional control voltage whose magnitude varies in accordance with the average strength of the signal to be demodulated by the diode 32. The

combined output of diodes 49 and 4| consists principally of noise components and is used for balancing out similar noise components appearing in the main signal channel.

As a result of the potential at conductor 59,

a current also flows to ground through resistors 38 and 39, this current consisting of both a direct current component and a superimposed signal frequency component. The signal frequency components are separated from the direct current component by capacitor 54 and applied via point A to either Fig. 2 or Fig. 3. By-pass capacitor 66 filters out components of carrier frequency.

Triode 55 is arranged as a signal frequency duction to cease-"in diode 30L aii'lplifier. Y 1 Anode' potentialsis suppliediromtermlnttl 55 conductor 51% through loadresistor 58.

The signal frequency output is-- taken through blocking capacitor- 59 to terminal 50 where it is available forfurther: amplificationor for such other' use- -as maybe desired; Grid bias is derived-"from fixed cathode resistor BI and adjust'able cathode resistor 62 and their associated by-pass capacitor 61. The gridisereturned to ground through input potentiometer --'63,-- the input b'e'ing applied to -poten-tiometer -63 through blocking capacitor 54'- which is connected via conductor-titofpoint=C.

"Considering vfirst the limiting action obtained using Fig! 2gthe'unidirectional con-trol voltage at point-B produces'a current which flows through diode- 1 in-its conductingdirection and through resistor 202--to ground. T he current' fiowingin diode'Zll I varies 'inmagnitude-in accordancewith variations in the average signal strength appearingatterminal I. Demodulated-voltagesofsignal frequency are applied via point A =across resistor 202 to ground. Negative peaks of noise will tend to dri-vepoint A sufficientlynegative to bring the anodeof-diode ZM-to the same-potenti-al -"as its cathode. At this point, conduction through' diode 20L will cease and any-further increase in-negative potential will be unable to reverse-the flow of control-current through diode Ml by virtue of the unidirectional conductivity ofi 'th'e= diode. This-produces a limiting-action the magnitude of the control current inaccord ance withdemodulatedsignal, these variations in current passing via: point C, conductor 55; blockin'g capacitor? and'potentiometer to the grid of t'riode -fi where'they are amplifiedas described above.

*Figi-fi -shows alimiter which may bemade symmetricaliin itsaotion and will suppress peaks of: either polarity with *equal efiectiveness; The control voltage produces a current which divides into two 'branches the current in one branch flo'wingto ground through diode 3M and adjustable resistor 303 *an'd'in the other branchthrough diode 302-- and' adiustable resistor 304.

1 The signal f-requency components from'point A-areapplied across resistor 303. The limited output is i derived from the potential drop" -across resistor 304. Negative peaks will cause con- Positive peaks will-be passed by diode I but will causeconduction toceasein-"diode 302. Values intermediatethese two limiting values will cause 'a .corresponding variation inthe current through resistor 304 and will. produce a signal outputin which peaks .of .either. polarity. are limited-at point .C which-will be. amplified by triode 5'5 as heretofore'de scribed.

A stabilized control voltage is provided s, by

the actionnoflv triode. which is -.connectedaas a cathode follower, :lts grid is connected through resistor? to the:output'ofldiodaafl, whichris neg atives with: respectdtorground. By-pass capacitor 15iremoves-3the signalcomponents appliedrto'ithe grid of triode 15 but is suificiently small in :capacitanc eL- value to avoid..undue sluggishness in responding :to changes in: signal: strength; shiredu'cedzsanode voltage issupplied from terminal 6Brandxthe.potentiometerarrangement comprising resistors sa'randln. .-A$::-the grid-of triode 35 becomes more negative in response to increased strength of demodulatedsignal .output by diode 32,v conduction through .triode 35 decreases and its cathode becomes. less positive by reason of the reduction .in anode current. A resistor -lfiiand filter: capacitor 1 1 provide further. filtering of. the control voltage appearing at the: cathode. of tr-i ode :35 before this-control voltage is applied-to terminal 1|. This .filtering may be omitted-x ii not required, or may beprovided in other circuits 1 which utilize the control voltage.

Becoming less positive is-equivalent tobecomingmore negative, and hencethe automatic'v'olume .control: :circuits, .in 'whichlthe gain :of amplifier and converter stages .may be: controlled byvarying thegridibias of the tubes anincreaseuin filtered by resistor 14 and capacitor 15 and is applied to the control grid of pentode 12. In the absence of a signal, or if the signal is too weak; arelatively heavy-current flows through pentode l2 and-through the commoncatho'de resistors GI and 62'which-also serveoutputtriode 55. This renders the cathode of 'triode Sui-- ficiently positive'with respect to its grid to cut oficurrent=fiow in triode 55 thus silencing the output circuit.

When a signal is received,- conductor 'Hl'becomes more'negative, as previously explained.

Conduction through pentode 12 *is 'reducedand the cathode of triode 55 is restored to a less positive potential permitting it again to operate as an output'tube.

In operation, a carrier frequency input is received at terminal l, amplified. by pentode 3 and demodulated by diode 32, the band pass action of tuned transformer l2 confining effectiveness to frequencies intheinput frequency ranges Noise-frequencies, amplified by pentodes-d. and 5 and selected in the higher and .lowergportions of theinput frequency range by tuned. transformers. 20 and 23, respectively-rare. demodulated by diodes and 4!, respectively, and-theircombined output is applied with polarity opposite to.that of-the main signal channel .to produce a reduction in'noise by balancing out-noise-components appearing in the signal channel output.

The. remainig noise is further :reducedbyi a peak limiter whose operating limits are controlled in accordance'with-the average signal strengthby a control voltage produced in the noisebalancin'g circuits. This: control voltage "is: amplified: and

is available. for. automatic volume control; The

Inthis "manner, increased automatic ,volume control voltage operates a squelch control circuit which cuts off the output amplifier tube in the absence of a received signal.

I have described what I believe to be the best embodiment of my invention. I do not wish, however, to be confined to the embodiment shown, but what I wish to cover by Letters Patent, is set forth in the following. claims.

I claim:

1. In a noise reducing circuit, a signal channel for demodulating frequencies within a predetermined range, a noise channel for demodulating frequencies within, a range overlapping the first-named range, balancing meansfor combining the output of the signal channel and the noise channel in opposing relationship to reduce noise components appearing at the output of the signal channel, means coupled with the signal channel for producing a control voltage whose magnitude varies in accordance with the aver age intensity of the signal to be demodulated, peak limiting means in the signal channel output and responsive to the control voltage for varying the maximum permissible signal peak voltage directly in accordance with the magnitude ,of the control voltage and output means responsive to the signal channel output subject to the action of both the noise balancing means and the peak limiting means.

2. In a noise reducing circuit, a signal channel for demodulating frequencies within a predetermined range, a noise channel for demodulating frequencies within a range differing appreciably from the first-named range, balancing means for combining the output of the signal channel and the noise channel in opposing relationship to reduce noise components appearingat the output of the signal channel, means coupled with the signal channel for producing a control voltage whose magnitude varies in accordance with the average intensity of the signal to be demodulated, peak limiting means in the signal channel output and responsive to the control voltage for varying the maximum permissible signal peak voltage directly in accordance with the magnitude of the control voltage and output means responsive to the signal channel output subject to the action of both the noise balancing means and the peak limiting means.

3. In a device for reducing the effect of noise or other disturbances appearing in the demodulated output of a tuned carrier frequency amplifier having a band pass characteristic, a rectifier circuit responsive to frequencies in the central portion of the pass band of the tuned amplifier, a second rectifier circuit responsive to frequencies in the upper portion of the pass band of the tuned amplifier, a third rectifier circuit responsive to frequencies in the lower portion of the pass band, coordinating means combining the outputs of the two last named rectifier circuits in differential relationship with the output of the first named rectifier circuit to produce a combined output in which noise components appearing at or near the limits of the pass band have appreciably reduced the effects of other noise components in the central portion thereof, filter means coupled with said first named rectifier circuit for producing a unidirectional control current whose magnitude will be substantially directly proportional to the average signal intensity, a circuit including a fourth and fifth rectifier connected with said filter means for energization simultaneously each in its conductive direction by the filtered controlcurrent,

means for supplying signal frequency current to the circuit of the fourth and fifth rectifiers to vary the instantaneous ratio of division of the direct current flowing between these two rectifiers and for applying peaks of signal frequency voltage in either direction to decrease the current flow in either the fourth or the fifth rectifier, and for suppressing peaks of signal voltage equalling or exceeding that value which will reduce the current at either the fourth orthe fifth rectifier to zero, and means for amplifying the signal frequency currents thus limited.

4. In a device for reducing the effect of noise or similar disturbances in the output of a tuned amplifier having a band pass characteristic, 9, group of three tuned amplifier detectors coupled with the output of the tuned amplifier, one of the group having its center frequency tuned to be substantially identical to the center frequency of the pass-band of the tuned amplifier-and the turning of the other two amplifier detectors be: ing higher and lower respectively than the cen-, ter frequency by substantially the same appreciable amount, coordinating means connecting the combined outputs of the two last-named amplifier detectors to oppose the output of the firstnamed amplifier detector in such degree and in. such phase relationship that noise components appearing at or near the limits of the pass-band of the tuned amplifier tend to oppose and thereby reduce the magnitude of other components relatively nearer the central portion of the passband, means coupled with the coordinating means for deriving a unidirectionally demodulated current preponderantly produced by the first named amplifier detector, filter means coupled to the last named means to remove thehigh frequency components therefrom thus to,

will be substantially directly proportional to the.

average signal intensity, a circuit including a pair of rectifiers connected with said additional filter means for energization simultaneously in.

their conductive directions by the filtered control current, means for applying the signalfrequency components of the unidirectionally demodulated current to the circuit of the pair of.

rectifiers to vary the instantaneous ratio of division of the direct current flowing between the two rectifiers of the pair and for applying peaks of signal frequency voltage of either polarity to.

decrease the current flow through one or the other of the rectifiers of the pair and for suppressing peaks of signal frequency voltage equalling or exceeding that value which will reduce.

either one or the other of the rectifier currents to zero, and means for amplifying for reproduction the signal frequency components thus limited.

5. In a device for demodulating a carrier frequency current amplitude-modulated by currents of signaling frequencies, noise reducing means comprising a first demodulating means tuned to respond principally to frequencies in the imme-f.

diate range of the carrier frequency, a second demodulating means tuned to respond principally I to frequencies in a range differing appreciably from the range of response of the first demodulating means, combining means coupled with the outputs of the first and second demodulating means in differential relationship to reduce the effects of noise components demodulated in like tausouoo from the varying unidirectional; voltage'to 'produce aunidirectional control current whose average magnitude varies ins-accordance :with the average'amplitude of the carrier frequency current to'be'demodulated, rectifier means connected to prevent reversal of the: controlicurrent; further filter (means icoupled with'therrcombining: means 1 for separating the idemodulated'ssignalz frequency currents from the varying unidirectional :voltage "yielded byz'thecombiningmeans,xmeansrcoupled with the output of "the: further:filter zzmeansifor varying the instantaneous magnitude of thetcontrol: currentin 1 accordance with the demodulated signal frequency components, and output means responsive to the last namedvariations in instantaneous current magnitude.

6. In a device for demodulating'a carrier-frequency currentamplitude-modulated by currents of signaling frequencies, noise reducing means comprising a first domodulating means tun'd'to "respond principally tofrequencies in the 'immediate range of the carrierfrequency, a seconddemodulating means tunedto respond principally to frequenciesin-arange differing appreciably from the range ofresponseof thefirst demodulating meansp-combiningmeans coupled to-the-outputs of thefirst and-second demodulating means in differential relationship' toreduce thereifects or noisecomponents-demodulated in like phase and like amplitude by--both -demodulationmeans and for yielding aunidirectional voltage varying in instantaneous amplitude in accordanc e with the demodulated signal frequency currents, filter meanscoupledwith the combining means for removing l the signal frequency components from the varying unidirectionalvoltage to produce a unidirectional control current whose average ma nitudevaries in accordance-'with thehaverage amplitude of the---carrier-frequency current to be demodulated, rectifier means connected to prevent reversal of the control current, further filter means coupled with the combining means for separating the demodulated signal frequency currents from the varying unidirectional voltage yielded by the combining means, means coupled with the output of the further filter means for varying the instantaneous magnitude of the control current in accordance with the demodulated signal frequency components, output means responsive to the last-named variations in instantaneous current magnitude, and squelch control means responsive to a predetermined degree of diminution in the magnitude of the control current for suppressing operation of the output means.

'7. In a device for demodulating acarrier frequency current amplitude-modulated by currents of signalling frequencies, noise reducing means comprising a first demodulating means tuned to respond principally to frequencies in the immediate range of the carrier frequency, a second demodulating means tuned to respond principally to frequencies in a range differing appreciably from the range of response of the first demodulating means, combining means coupled with the outputs of the first and second demodulating means in differential relationship toreduce the effects of noise components demodulated in like phase and like amplitude by both demodulation means-and foryielding a unidirectional voltage varying in instantaneous amplitude in accordance with the demodulated signal frequency currents, filter means coupled with the combining means for removing the signal frequency components from'the'varying unidirectional voltage to produce a control current whose average magnitude'varies in accordance with the average amplitude of the carrier frequency current to be demodulated, rectifier,means connected to prevent reversal of the control current, further filter means coupled with the combining means for separating the demodulated signal frequency signal frequency components, output means responsive to the lastnamed variations in instantaneous current magnitude, and squelch control means--responsive to a predetermined degree of diminutionginthewmagnitude of the unidirectional voltage :fori'suppressing operation-of the .outputmeans.

,8.-In :a'devicei for demodulating a carrier frequency current amplitude-modulated by currents of "signaling frequencies, noise reducing means comprising a-first demodulating means-tuned to res ondprincipally to frequencies in the immediateura-nge of the carrier frequency, a second demodulating means. tuned to respond principally to frequencies in a rangehigher than'andidiifering-appreciablyfrom the'range of response- 0f the? first "demodulating means, a third .demodulating means tuned to respond-principally. to frequencies in a range lower than and'diifering' ap- 'preciably from theJrange of response of the first demodulating means, combining means coupled with thetoutput of'the first demodulatingmeans with the'cumulatively combined'outpu'ts of: the second. and third demodulatingimeans in differential relationship to reducethe-effects ofnoise components demodulated in like *phase and like amplitude: .tby bothdemodulation-means and for yielding; a unidirectional voltage varying in tinstantaneous amplitude in accordance with the demodulated signal frequency currents, filter means coupled with the combining means for removing the signal frequency components from the varying unidirectional voltage to produce a unidirectional control current whose average magnitude varies in accordance with the average amplitude of the carrier frequency current to be demodulated, rectifier means connected to prevent reversal of the control current, means for varying the instantaneous magnitude of the control current in accordance with the demodulated signal frequency components, and output means responsive to the variations in instantaneous magnitude.

9. In a device for demodulating a carrier frequency current amplitude-modulated by currents of signaling frequencies, noise reducing means comprising a first demodulating means tuned to respond principally to frequencies in the immediate range of the carrier frequency, a second demodulating means tuned to respond principally to frequencies in a range differing appreciably from the range of response of the first demodulating means, combining means coupled with the outputs of the first and second demodulating means in differential relationship to reduce the effects of noise components demodulated in like phase and like amplitude by both demodulation means and for yielding a unidirectional voltage varying in instantaneous amplitude in accordance with the demodulated signal frequency currents, filter means coupled with the combining'means for removing the signal frequency components from the varying unidirectional voltage to produce a unidirectional control current whose average magnitude varies in accordance with the average amplitude of the carrier frequency current to be demodulated, rectifier means connected to prevent reversal of the control current, means for varying the instantaneous magnitude of the control current in accordance with the demodulated signal frequency components, and output means responsive to the last-named variations in instantaneous magnitude. 10. In a. device for reducing the efiect of nois or other disturbance appearin in the demodulatedoutput of a tuned carrier frequency signal amplifier having a band pass characteristic, a plurality of individual amplifiers having a common input coupled with the output of the tuned amplifien'a tuned transformer coupledwith'output of each individual amplifier the resonant frequency of one tuned transformer being'sub's'tantially identical with the carrier frequency and the resonant frequencies of every other tuned transformer differing appreciably from 'that' f the first-named tuned transformer, demodulating means connected with the output of each tuned transformer, the polarity of the output of the demodulating means connected with firstnamed tuned transformer being instantaneously opposite to that of each of the other demodulating means, adjustable means for varying the relative amounts of amplification ofithe several individual amplifiers, combining means for combining the outputs of the plurality demodulating means to reduce the effect of noise components appearing in the output of the first-named demodulating means by balancing it against noise components appearing in the outputof another demodulating means, peak voltage limiting means coupled with the combined outputs of the demodulating means and having a variable threshold below which no limiting action occurs,

12 means coupled with the combining means responsive to the average intensity of the signal to be demodulated and for varying the threshold of the 'peak voltage limiting means directly in accordance with the average intensity of the signal to be demodulated, and output means responsive to the demodulatedsignal after subjection to the action of the peak voltage limiting means.

11. The method of reducing noise in the demodulated output of a carrier frequency signal receiver comprising a tuned amplifier having a band pass characteristic which consists in balancing noise components appearing .in the central portion of the amplifier pass-band against noise components appearing in a different portion thereof and limiting the voltage peaks remaining after noise balancin to a magnitude which varies directly in accordance with the signal strength.

CARLTON WASMANSDORFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,559,743 De Bellescize Nov. 3, 1925 1,733,414 Knapp 001;. 29, 1929 1,959,275 McCaa May 15, 1934 1,991,277 Glasgow Feb. 12, 1935 2,179,974 Beers Nov. 14, 1939 2,197,516 Case Apr. 16, 1940 2,215,946 Radinger Sept. 24, 1940 2,244,639 Budelman June 3, 1941 2,273,098 Foster Feb. 17, 1942 2,279,819 Fyler Apr. 14, 1942 2,281,395 Travis Apr. 28, 1942 2,324,275 Becker July 13, 1943 2,337,005 Selby Dec. 14, 1943 2,343,115 Noble Feb. 29, 1944 2,422,976 Nicholson, Jr June 24, 1946 FOREIGN PATENTS Number Country Date 208,730 Great Britain Dec. 11, 1924 

